The present invention relates generally to specular proximity detectors adaptable for use in article handling equipment. More particularly, however, this invention is a specular proximity detector capable of sensing with precision the presence of articles at a selected location and an article handling apparatus capable of positioning articles at precise locations determined by the precision specular proximity detector.
Proximity detectors are widely used to determine the arrival of articles at precisely specified locations. Most determine the arrival by measuring a physical property of either the environment established at the location or of articles arriving at the location to obtain an indication of the distance separating the specified location from the detecting element of the proximity detector. For example, capacitive devices measure the electric field established in the region of the specified location. When an article having a known dielectric coefficient is positioned at the specified location, the established electric field is disturbed in a predictable manner. The predictable disturbance is utilized to indicate the arrival at the specified location of articles of known dielectric coefficient.
Ultrasonic devices measure ultrasonic energy either reflected from or transmitted through the specified location. When an article having a known ultrasonic energy absorption coefficient is positioned at the specified location, the reflected or transmitted ultrasonic energy is altered in a predictable manner. The predictable alteration is indicative of the arrival at the specified location of articles of known ultrasonic energy absorption coefficient.
Air jet devices establish a high velocity air stream through the specified location and the air flow is measured to obtain an indication of the arrival of articles at the specified location. When an article establishing a known air flow impedance is positioned at the specified location, the air flow through the location is altered in a predictable manner. The predictable alteration of the established air flow is utilized to indicate the arrival at the specified location of articles establishing a known air flow impedance in the path of the air flow.
Specular proximity detectors assure radiation reflected from the specified location. When an article having a known coefficient of reflectivity is positioned at the specified location, a predictable fraction of incident radiation is reflected to a radiation sensitive element, the predictable fraction indicating the arrival of the articles at the specified location.
While such proximity detectors are satisfactory for determining the arrival at a specified location of articles having a certain predictable property, they are unreliable when used to determine the arrival of articles whose property being measured is unpredictable or varies widely over a range. For example, semiconductor wafer handling apparatus are employed to facilitate the automatic manufacture of semiconductor materials and fabrication of semiconductor devices. Usually, such apparatus are required to manipulate wafers of a variety of sizes, shapes and semiconductor materials as they are conveyed through a series of testing and inspecting stations for the performance of quality control operations. In highly automated semiconductor manufacturing plants, the wafers are transported by a moving conveyor from a wafer loading station, through the testing and inspecting stations to a wafer unloading station. The wafers are sequentially loaded onto the conveyor from and unloaded from the conveyor to wafer magazines, commonly called "cassettes", each having the capacity to hold several wafers. Wafer magazines have come to be standardized. The standardized magazines store the wafers in a vertical stack so that they can be withdrawn from and deposited in the magazine by machine-controlled means when loading and unloading the conveyor. In one type of wafer handling apparatus, the magazines are located above the conveyor and the moving conveyor is employed to withdraw and deposit the wafers,. As the wafers are withdrawn from (or deposited in) a magazine, the magazine is moved incrementally along a line perpendicular to the conveyor, whereby each wafer is gently engaged by the moving conveyor to withdraw it (or is gently placed in the magazine by the moving conveyor to deposit it). To insure gentle handling of the delicate semiconductor wafers, the incremental movements of the magazines are carefully controlled by proximity detectors that function to detect the position of the magazines precisely relative to the moving conveyor. Proximity detectors of the kind referred to briefly hereinbefore require alignment whenever the wafer handling apparatus is required to manipulate wafers of different measured property. However, alignment of the proximity detectors does not avoid proximity detection errors resulting from normal variations in the measured property of semiconductor wafers of the same kind.
As will be described in detail hereinafter, the present invention utilizes a specular proximity detector capable of reliably sensing the presence of articles at a specified location, which articles may have a coefficient of reflectivity lying anywhere within a wide range. Generally, specular proximity detectors employed in semiconductor wafer handling apparatus have utilized infrared and visible light radiation projected through the site at which the wafers are to be detected. When a wafer arrives at the site, the projected radiation is reflected to a photosensitive transducer, which responds by providing an indication of the arrival of the wafer, hence, wafer magazine at the desired position for loading or unloading wafers with respect to the moving wafer conveyor.
One typical infrared proximity detector employs an infrared emitter and cooperating beam-forming apertured colliminator positioned to allow a beam of infrared radiation of small divergence to be projected along a path defining a selected angle of incidence at its intersection with the potential site of the wafers to be detected. An infrared-sensing transducer and cooperating mask are positioned along a line in the incident plane defining the angle of reflection at the intersection of the incident beam and potential wafer site.
Such proximity detector is characterized by numerous disadvantages. A majority of the radiation generated by the infrared emitter is intercepted by the collimating structure and wasted in the process of collimating the beam, making the proximity detector inefficient. In addition, the apertured collimator only partially collimates the infrared beam. Consequently, the formed incident infrared beam and the reflected beam each are in the form of a diverging cone. This beam divergence and the reflected beam acceptance angle defined by the mask position along the reflected beam line limits the resolution of distance discrimination of the proximity detector. Variations in wafer surface finish produce an additional negative effect. A relatively non-reflective surface reflects less energy into the beam acceptance angle defined by the mask positioned along the reflected beam line than a highly reflective surface. As a result, the response of the infrared-sensing transducer varies widely with the typical variation in coefficient of reflectivity of the various semiconductor materials from which wafers are made. The output of such transducers has been found to vary by a factor of 600 to 1 for semiconductor wafers of different common surface properties and coatings. Assuming such proximity detector is aligned to provide an indication of wafers of average coefficient of reflectivity being positioned at the desired site, wafers having a highly reflective surface will produce a false indication of such positioning when the wafers are farther from the reflected beam sensor. On the other hand, wafers having a relatively non-reflective surface will not reflect sufficient energy to produce even a false indication regardless of their distance from the reflected beam sensor.
In semiconductor wafer handling apparatus where proximity detectors are employed to control the positioning of wafer magazines relative to a moving wafer conveyor, such false or failure of indications cause mispositioning of the magazines. Usually, the delicate semiconductor wafers are damaged or destroyed during wafer withdrawal or insertion operations attempted with mispositioned magazines. Such false or failure of indication can be avoided only by adjusting the alignment of the proximity detector for wafers of different ranges of coefficient of reflectivity. Besides complicating the use of the apparatus employing such proximity detectors, the reflectivity of semiconductor wafers of a particular material composition and configuration is not uniform, varying enough to cause occasional false or failure of indication of the presence of a wafer at a selected location.
It has been proposed to incorporate a point-focus lens system in specular proximity detectors to avoid the proximity detection unreliability growing out of the aforementioned disadvantages. In such detectors, a convex converging lens or series of lenses is positioned to focus the emitted radiation at a point at the site at which the wafers are to be detected. While the proximity detection reliability is improved over those aforedescribed apertured collimating specular proximity detectors, the use of a point-focus lens system has a major drawback. Very accurate optical alignment is necessary in specular proximity detectors employing a point-focus lens system to assure that the spot of maximum beam intensity is coincident with the desired point of sensitivity of the reflected radiation sensor. The necessity of accurate optical alignment complicates the manufacture and installation of the proximity detector, and adds to the expense of their manufacture and installation. Furthermore, such proximity detectors are subject to becoming misaligned frequently when used in environments such as found in automated semiconductor wafer handling apparatus where considerable movement of mechanical parts and operator manipulation of apparatus components and handled wafers occur.